A gas turbine aircraft engine comprises of a compressor compressing ambient air, a combustor burning fuel together with the compressed air and a turbine for powering the compressor. The expanding combustion gases drive the turbine and also result in thrust for propulsion.
Aircraft flying at high altitude ingest the clean air prevailing at these altitudes. However, at the aerodromes the air contains foreign particles in form of aerosols which enters the engine with the air stream. Typical particles found in the aerodrome air are pollen, insects, hydrocarbons coming from industrial activities and salt coming from nearby sea. While the aircraft is grounded at the airport there are additional particles to consider such as combustion residues in engine exhaust from taxing aircraft, chemicals coming from aircraft de-icing and ground material such as dust. The majority of the foreign particles will follow the gas path through the engine and exit with the exhaust gases. However, there are particles with properties of sticking on to components in the engine's gas path, especially in the compressor section of the engine. This is known as fouling.
Compressor fouling results in a change in the properties of the boundary layer air stream of the compressor components. The presence of foreign particles results in an increase of the component surface roughness. As air flows over the surface the increase of surface roughness results in a thickening of the boundary layer air stream. The thickening of the boundary layer air stream has negative effects on the compressor aerodynamics in form of a reduced mass flow. At the blade trailing edge the air stream forms a wake. The wake forms a vortex type of turbulence with a negative impact on the air flow. The thicker the boundary layer the stronger the turbulence in the wake and the more it reduces the mass flow. Further, a thick boundary layer and a stronger trailing edge turbulence result in a reduced compression gain which in turn results in the fouled compressor compressing air at a reduced pressure ratio. Anyone skilled in the art of heat engine working cycles understands that a reduced pressure ratio result in a lower thermal efficiency of the engine. The compressor fouling not only reduces the mass flow and pressure gain but also reduces the compressor isentropic efficiency. Reduced compressor efficiency means that the compressor requires more power for compressing the same amount of air. The power for driving the compressor is taken from the turbine via the shaft. With the turbine requiring more power to drive the compressor there will be less power to create thrust for propulsion. For the aircraft pilot this means he must throttle for more fuel as to compensate for the reduced thrust. Throttling for more fuel means the consumption of fuel increases and thereby increasing the operating costs. The performance loss caused by compressor fouling also reduces the durability of the engine. As more fuel has to be fired for reaching a required thrust level, follows an increase in the engine firing temperature. When the pilot on the runway throttles for take-off, the engine's hot section components are under critical high temperature load. Controlling the combustion gas temperature is a key issue in engine performance monitoring. The controlling temperature known as exhaust gas temperature (EGT) is measured with sensors in the gas path downstream of the combustor outlet. The EGT is carefully monitored by logging both temperature and exposure time. During the lifetime of the engine the EGT log is frequently reviewed. At a certain point it will be required that the engine is taken out of service for an overhaul where hot section components are inspected and replaced if required.
Compressor fouling also has a negative effect on the environment. The difference in fuel consumption of a virgin engine delivered from the factory and an engine with a fouled compressor may typically be 1%. With the increase of fuel consumption follows an increase of emissions of green house gas such as carbon dioxide. Typically combustion of 1 kg of aviation fuel results in formation of 3.1 kg carbon dioxide. Further, high combustor temperature has a negative effect to the environment. With the increase of firing temperature follows an increase of NOx formation. NOx formation depends to a large extent on the design of the burner and a general number can not be provided. However, any incremental temperature rise to a given burner design results in an increase in NOx formation. Hence, compressor fouling has negative effects to aero engine performance such as increasing fuel consumption, reducing engine life and increasing emissions.
A number of engine washing techniques has developed over the years as to reduce or eliminate the negative effects of fouling. The simplest washing method is taking a garden hose and spraying water into the engine inlet. This method has however limited success due to the simple nature of the process. An alternative method is hand scrubbing the blades with a brush and liquid. This method has limited success as it does not enable cleaning of the blades in the interior of the compressor. Moreover, it is time-consuming. U.S. Pat. No. 5,868,860 to Asplund discloses the use of a manifold for washing of aero engines. Further the patent discloses the use of high liquid pressure as means of providing a high liquid velocity, which together with rotation of the engine shaft will enhance the cleaning efficacy. U.S. Pat. No. 6,394,108 to Butler discloses a thin flexible hose which one end is inserted from the compressor inlet towards the compressor outlet in between the compressor blades. At the inserted end of the hose there is a nozzle. The hose is slowly retracted out of the compressor while liquid is being pumped into the hose and sprayed through the nozzle. However, the washing efficacy is limited by the compressor rotor not being able to rotate during washing. Despite existing wash technologies and patents there is a need for new technologies enabling practical washing to be conducted in a less labour intensive, low cost, simple and safe way.